The present invention relates to the field of signal processors and more specifically to methods and apparatus for minimizing the distortion caused by non-linear elements, such as amplifiers, in wireless communications systems.
Distortion
Non-linear elements such as amplifiers in communications systems inherently provide distorted output signals that are not simply linear functions of the input signals. Distortion can be reduced by separating wanted in-band signal components from unwanted out-of-band signal components. Such separation is particularly effective for narrow band signals where filters can be used to remove unwanted signal components. For a communications system that operates only on a single (in-band) channel, the out-of-band signal components generated by an amplifier can be easily removed by filtering or can be ignored if they are non-interfering. However, in communications systems that use signals that extend over a broad band of frequencies (broadband signals), the separation of in-band and out-of-band signals is more difficult. In communications systems that operate with a number of adjacent channels in adjacent frequency bands, the out-of-band components for one channel frequently exist in the same frequency spectrum as (and interfere with) the in-band signals of adjacent channels.
Cellular Systems
Present day cellular mobile telephone systems operate with many adjacent frequency channels and operate in an environment where elimination of signal distortion is important. Cellular mobile telephone systems "reuse" frequency within groups of cells to provide wireless two-way radio frequency (RF) communication to large numbers of users. Each cell covers one geographic area and collectively groups of adjacent cells can cover a larger geographic region. Each cell is allocated a fraction of the total amount of RF spectrum for cellular users located in the cell.
In cellular systems, typically each cell has a base station with radio frequency (RF) transmitters and RF receivers co-sited for transmitting and receiving communications with cellular users. The base station employs forward RF frequency bands (carriers) to transmit forward channel communications to users and employs reverse RF carriers to receive reverse channel communications from users in the cell. Conventional forward channel communications employ fixed power, at fixed frequencies and have fixed sectors if sectorized antennas are used.
The forward and reverse channel communications use separate frequency bands to enable simultaneous transmissions in both directions using frequency domain duplex (FDD) signaling. Time domain duplex (TDD) signaling, in which the forward and reverse channels take turns using the same frequency band, is possible.
In cellular systems, particular channels are allocated to individual users. Each user=3 s communications are routed by the system through the channel allocated to that user. Signals broadcast by the system must be carefully regulated so that they remain within the allocated channels. "Out-of-band" signals produced by intermodulation signal components from one channel can cause unacceptable interference with communications in other channels. Intermodulation products are produced through the interaction of two or more wanted signals in non-linear system elements. For example, in a multiband 40-channel communications system, thousands of intermodulation products may exist. Cellular communications systems, therefore, place stringent restrictions on out-of-band signal emissions in order to minimize channel-to-channel interference.
Distortion Reduction
In order to reduce the out-of-band signal emissions from one channel into another, many different methods have been proposed. As one example, feedback systems have been proposed in which a portion of the amplifier output signal is processed and fed back to alter the input signal to the amplifier. Feedback systems, however, are expensive and have not achieved entirely satisfactory results. As another example, predistorting systems have been proposed to predistort the input signal to an amplifier with a predistortion transformation which is complementary to the distorting transformation of the amplifier. The predistorted input signal is intended to produce an undistorted output signal that is a linear function of the input signal. su Known predistortion techniques, however, have primarily operated on the wanted signal components and have not satisfactorily processed unwanted intermodulation signal components.
One prior art predistortion method is described in U.S. Pat. No. 4,462,001 entitled BASEBAND LINEARIZER FOR WIDEBAND, HIGH POWER, NONLINEAR AMPLIFIERS. In that patent, separate look-up tables containing amplitude and phase correction factors are provided containing a multiplicity of entries which define predistortion transformation parameters appropriate for use with a corresponding multiplicity of different input signals. In operation, the fluctuating power level of the input signal to be amplified is continuously measured and used to address the look-up tables to obtain corresponding predistortion parameters to predistort the input signal before it is input to the amplifier.
Another prior art method is described in U.S. Pat. No. 4,700,151 entitled MODULATION SYSTEM CAPABLE OF IMPROVING A TRANSMISSION SYSTEM. In that patent, the real and imaginary quadrature components of the input signal sample are used to index a look-up table containing more than a million entries of predistortion transformation parameters. The look-up table entries are adaptively changed in response to variations in the amplifier's distorting characteristics. If the channel is changed, as is common in cellular systems, every entry in the look-up table must be updated. This updating process is burdensome and often takes a longer time than is available. This technique is used to modify the wanted signal rather than the intermodulation components.
Still another prior art method is described in U.S. Pat. No. 5,049,832 entitled AMPLIFIER LINEARIZATION BY ADAPTIVE PREDISTORTION. In that patent, stored table entries in rectangular coordinate format are provided to enable the subsequent predistortion operation to be performed more simply than in the U.S. Pat. No. 4,700,151 patent with a smaller look-up table. Again, this technique is used to modify the wanted signal, not the intermodulation components.
The known methods, like those discussed above, primarily address the wanted signals, not the intermodulation components, so that the results obtained are not entirely satisfactory and particularly are not satisfactory for broadband systems having many adjacent frequency channels.
In broadband systems, even though the adverse effects of intermodulation distortion are known, amplifiers have not been able to adequately overcome the intermodulation distortion problems. To date, even the best amplifiers achieve only 20 to 40% efficiency for broadband applications. This low efficiency is in large part due to the third order intermodulation products.
The intermodulation products are described, for example, in the article entitled "Intermodulation Distortion in a Multi-Signal Environment," by Michael Leffel, RF Design, pages 78-84, Jun. 1995, which describes the intermodulation caused by multiple input tones. The article predicts the behavior of amplifiers having multiple input tones based upon two-tone intermodulation parameters.
Although predistortion has been proposed to improve the operation of amplifiers, in actual practice, there is much need for improved predistortion methods and apparatus for broadband applications such as cellular systems. Although intermodulation distortion products have been identified as the cause of problems, the known signal processors have not been satisfactory for reducing those problems in broadband systems.
In accordance with the above background, there is a need for improved signal processors for use in broadband communications systems.